(1) Field of the Invention
The present invention relates to a process for the production of a lanthanum chromite film.
(2) Related Art Statement
Recently, fuel cells have attracted attention as power generators. The fuel cell is a device capable of directly converting a chemical energy possessed by a fuel to an electric energy, and is free from the limitation of the Carnot cycle. Thus, the fuel cell essentially has a high energy conversion efficiency, and a variety of fuels such as naphtha, natural gas, methanol, coal-reformed gas and heavy oil can be used. Further, the fuel cell produces of low pollution, and its power-generating efficiency is not influenced by a scale of a plant. Therefore, the fuel cell is an extremely promising technique.
In particular, since the solid oxide fuel cell (SOFC) operates at a high temperature of around 1,000.degree. C., electrode reaction is extremely active, and thus the fuel cell does not need a catalyst of a noble metal such as precious platinum. In addition, polarization is small, and output voltage is relatively high, and the energy-converting efficiency of the SOFC is higher as compared with other fuel cells. Furthermore, since all the constituent materials are solid, the SOFC is stable, and has a long use life.
In such an SOFC, a fuel electrode and an air electrode of adjacent SOFC elements (cell units) are generally connected in series through an interconnector and a connection terminal. Therefore, it is particularly demanded that the interconnector is provided in the form of a thin film to reduce its electric resistance.
As a technique for producing a thin film of the interconnector, chemical vapor deposition (CVD) or electrochemical vapor deposition (EVD) may be employed. However, such depositions require large scale film-forming apparatuses, and a treating speed and an area where the deposition can be made are too small.
A process for the production of the solid oxide fuel cell by using plasma spraying possesses the merits that the process has a high film-forming speed, is simple, and can form a thin and relatively dense film. For this reason, this process has been conventionally employed (Sunshine Journal, vol. 2, No. 1).
For example, a process is known in which a raw material for use in plasma spraying (hereinafter referred to as "plasma spray raw material") is prepared by solid solving cerium oxide or zirconium oxide and an oxide of a metal such as an alkaline earth metal or a rare earth element, and a solid electrolyte film is formed by plasma spraying this raw material after the grain size is adjusted (Japanese patent application Nos. 61-198,569 and 61-198,570).
However, since the porosity of the plasma sprayed film is generally great, gas tightness is insufficient for the interconnector of the SOFC. Cracks or layered defects occur in the film during the plasma spraying. Accordingly, a fuel such as hydrogen or carbon monoxide leaks through the interconnector during the operation of the SOFC, so that the electromotive force per cell unit of the SOFC becomes lower than, for example, 1 V in the ordinary case, output drops, and the conversion rate from the fuel to the electric power decreases.
It may be considered that such a leakage of the fuel is compensated by increasing the thickness of the film interconnector. However, in this case, the resistance of the cell becomes greater, and the output of the cell drops. Therefore, a process is desired, which can densify the interconnector, and can make the interconnector as thin as possible provided that no fuel leakage occurs, and can increase the output of the cell.
It is published in Cell Debate, September, 1991, page 205 that lanthanum calcium chromite is plasma sprayed onto a surface of a substrate made of lanthanum manganite, and then a dense lanthanum chromite film is produced by heat treating the plasma sprayed film, and that this dense film is used as an interconnector of an SOFC.
Before this publication, NGK Insulators, Ltd. developed a technique for densifying a lanthanum chromite film by heating a plasma sprayed lanthanum chromite film, and filed a patent application for this technique (Japanese patent application No. 3-25,245 filed on Jan. 28, 1991).
However, the present inventors have further examined this technique, and found out a new problem.
That is, it is found out that when the lanthanum chromite is formed by plasma spraying, a part of a chromium component is evaporated during the plasma spraying so that the chromite film suffers deficiency of the chromium component. Consequently, lanthanum oxide or calcium oxide precipitates in the plasma sprayed film to increase electric resistance. Further, since lanthanum oxide has remarkably high hygroscopicity, lanthanum oxide is chemically changed to lanthanum hydroxide in the sprayed film through absorption of moisture. This chemical change is accompanied by a volume change to break the lanthanum chromite film. For this reason, the lanthanum chromite film in which lanthanum oxide or calcium oxide is precipitated cannot be practically employed.